The present invention is directed generally to systems for the cancellation of radar clutter, and more particularly to a radar clutter cancellation system formed with a minimum of hardware.
A radar echo return usually contains both the target signal and a clutter signal. The clutter signal arises from reflections from stationary and slow moving background objects, i.e., rain, chaff, land, and is usually much stronger than the target signal. This unwanted clutter, however, can be discriminated against by the use of a clutter cancelling system. A clutter cancelling system operates on the principle that the moving target has a doppler frequency shift, but that the stationery or slow moving clutter has none or very little. Typically, such clutter cancelling systems are designed in one of two ways. The first way is to configure the system to divide the doppler frequency space into two regions--a stop-band region and a pass-band region. The doppler frequency space comprises the frequencies between 0 and the pulse repetition frequency. By properly choosing the width and location of the system's stop-band regions, it is possible to effectively reduce the clutter noise to an acceptable level. This type of system is generally referred to as an MTI canceller. The second type of system utilizes a plurality of frequency filters to divide the doppler frequency space into many narrow regions. Each filter corresponds to one of these narrow frequency bands. This type of filter is generally referred to as a doppler filter and is formed typically by an FFT (Fast Fourier Transform) network. This doppler filter not only can be used to discriminate against the unwanted clutter, but can also be used to resolve the target doppler frequency and to provide improvement against noise. Recently-developed doppler filter systems employ adaptive doppler filter banks and require large numbers of digital calculations to be made in a short time. The amount of hardware involved to implement these systems and the speed at which this hardware must operate tend to make these systems impractical.